Barium iodide and strontium iodide crystals and scintillators implementing the same

ABSTRACT

In one embodiment, a material comprises a crystal comprising strontium iodide providing at least 50,000 photons per MeV, where the strontium iodide material is characterized by a volume not less than 1 cm 3 . In another embodiment, a scintillator optic includes europium-doped strontium iodide providing at least 50,000 photons per MeV, where the europium in the crystal is primarily Eu 2+ , and the europium is present in an amount greater than about 1.6%. A scintillator radiation detector in yet another embodiment includes a scintillator optic comprising SrI 2  and BaI 2 , where a ratio of SrI 2  to BaI 2  is in a range of between 0:1 and 1.0, the scintillator optic is a crystal that provides at least 50,000 scintillation photons per MeV and energy resolution of less than about 5% at 662 keV, and the crystal has a volume of 1 cm 3  or more; the scintillator optic contains more than about 2% europium.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/255,375, filed Oct. 21, 2008 and entitled “BARIUM IODIDE ANDSTRONTIUM IODIDE CRYSTALS AND SCINTILLATORS IMPLEMENTING THE SAME,”which in turn claims priority to Provisional U.S. application Ser. No.60/988,475 filed on Nov. 16, 2007, from each of which priority isclaimed and each of which is herein incorporated by reference.

The United States Government has rights in this invention pursuant toContract No, DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to scintillator crystals, and moreparticularly to ionic iodide-containing crystals and scintillatordetectors employing the same.

BACKGROUND

Detection and classification of gamma ray emitters has attainedheightened importance in the protection of vulnerable targets andpopulaces from high energy explosives. Many nuclear explosives emitgamma rays, due to radioactive decay of the materials comprising theexplosives. However, many less harmful and non-explosive materials alsoemit gamma rays. Therefore, it is desirable to be able to identify, andwhenever possible, distinguish between the types of gamma ray emittersin an unknown material, possibly further concealed inside of a containeror vehicle of some type, such as a car, van, cargo container, etc.

Many types of materials emit gamma rays that appear very close togetheron a gamma spectrograph. Scintillator detectors use crystals that emitlight when gamma rays interact with the atoms in the crystals. Theintensity of the light emitted can be used to determine the type ofmaterial that is emitting the gamma rays. Scintillator detectors mayalso be used to detect other types of radiation, such as alpha, beta,and x-rays. High energy resolution scintillator detectors are useful forresolving closely spaced gamma ray lines in order to distinguish betweengamma emitters producing closely spaced gamma ray lines.

Detection sensitivity for weak gamma ray sources and rapid unambiguousisotope identification is principally dependent on energy resolution,and is also enhanced by a high effective atomic number of the detectormaterial. Generally, gamma ray detectors are characterized by theirenergy resolution. Resolution can be stated in absolute or relativeterms. For consistency, all resolution terms are stated in relativeterms herein. A common way of expressing detector resolution is withFull Width at Half Maximum (FWHM). This equates to the width of thegamma ray peak on a spectral graph at half of the highest point on thepeak distribution.

The relative resolution of a detector may be calculated by taking theabsolute resolution, usually reported in keV, dividing by the actualenergy of the gamma ray also in keV, and multiplying by 100%. Thisresults in a resolution reported in percentage at a specific gamma rayenergy. The inorganic scintillator currently providing the highestenergy resolution is LaBr₃(Ce), with about 2.6% at 662 keV, but it ishighly hygroscopic, its growth is quite difficult and it possessesnatural radioactivity due to the presence of primordial ¹³⁸La thatproduces betas and gamma rays resulting in interference in the gamma rayspectra acquired with LaBr₃(Ce). Therefore, it is desirable to have ascintillator detector that is capable of distinguishing between weakgamma ray sources that is more easily grown while still providing highenergy resolution.

SUMMARY

In one embodiment, a material comprises a crystal comprising strontiumiodide providing at least 50,000 photons per MeV, where the strontiumiodide material is characterized by a volume not less than 1 cm³.

A scintillator radiation detector according to another embodimentincludes a scintillator optic comprising europium-doped strontium iodideproviding at least 50,000 photons per MeV, where the europium in thecrystal is primarily Eu²⁺, and the europium is present in an amountgreater than about 1.6%.

A scintillator radiation detector in yet another embodiment includes ascintillator optic comprising SrI₂ and Bah, wherein a ratio of SrI₂ toBaI₂ is in a range of between 0:1 and 1.0, where the scintillator opticis a crystal that provides at least 50,000 scintillation photons per MeVand energy resolution of less than about 5% at 662 keV, the crystal hasa volume of 1 cm³ or more, and the scintillator optic contains more thanabout 2% europium.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates two plots of relative intensity versus wavelength forseveral scintillator samples.

FIG. 2 is a plot of relative intensity versus wavelength for fivescintillator samples.

FIG. 3 is a plot of counts versus wavelength for undoped SrI₂ in pureand impure forms.

FIG. 4A is a plot of gamma ray spectra acquired with LaBr₃(Ce), SrI₂(5%Eu), and NiI(Tl) scintillators of the ¹³³Ba source.

FIG. 4B is a plot of gamma ray spectra acquired with LaBr₃(Ce), SrI₂(5%Eu), and NiI(Tl) scintillators of the ¹³⁷Cs source.

FIG. 5 is a chart comparing eight different measured or calculatedcharacteristics for seven different scintillator materials.

FIG. 6 is a flow chart of a method according to one embodiment.

FIG. 7 illustrates three plots of relative intensity versus time forseveral scintillator samples.

FIG. 8 is a chart of energy resolution as a function of gamma ray energyacquired for SrI₂(Eu) and LaBr₃(Ce) crystals.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

Several crystalline iodides are known to function usefully asscintillators, including NaI(Tl), CsI(Na), CsI(Tl), and LuI₃(Ce).NaI(Tl) is by far the most common scintillator, being grown in largesizes by numerous companies and deployed in many commercial instruments.However, NaI(Tl) offers a modest light yield, which limits the gamma rayenergy resolution which is possible (about 40,000 photons/MeV). CaI₂(Eu)and LuI₃(Ce) both have higher light yields (about 70,000-100,000photons/MeV) but are exceedingly difficult to grow and the lattercontains Lu as a constituent (which is a natural beta-emitter leading toan undesirable background count rate). Interestingly, LaI₃(Ce) has alsobeen tested as a potential scintillator, but was found to benon-emissive at room temperature.

Two materials have been found to have great utility as high energyscintillators: SrI₂(Eu) and BaI₂(Eu), which may resolve the problemsfacing the related compounds LuI₃(Ce) and CaI₂(Eu). Other relatedcompounds currently being used as scintillators include Ce-doped LaCl₃and LaBr₃, which both have very high tight yields but have a loweratomic number (Z) than the iodides (Z of Cl, Br, I are 17, 35, 53,respectively)—the Z is a critical parameter since gamma photoelectricabsorption goes approximately as its fourth power, Z⁴. As a consequence,iodides are preferred as constituents in scintillators versus bromidesand chlorides, assuming other features are comparable.

In one embodiment, a scintillator detector makes use of SrI₂ or BaI₂crystals for the purpose of gamma ray detection, based on measuring theamount of scintillation luminescence generated by the material For thispurpose, the crystal may be doped or undoped, giving rise to excitonic(undoped), perturbed excitonic (e.g., Na, Mg, Ca, Sc or other doping ofelectronically inactive species), or activator luminescence (e.g., Eu²⁺,Ce³⁺, Pb²⁺, Tl⁺, Pr³⁺).

Undoped SrI₂ has a useful light yield, but its energy resolution withstandard photomultiplier tubes is only fair, due to its emission beinglong wavelength. When a Eu²⁺ activator is used, emissions in the blueregion are observed. Contrary to conventional wisdom and earlierfindings, a scintillator optic comprising SrI₂ doped with Eu, especiallyEu²⁺, has been found to provide a high energy resolution. For example, aSrI₂(Eu) crystal grown by the inventors evidenced an energy resolutionof <2.7% at 662 keV, challenging the performance of LaBr₃(Ce) obtainedunder the same conditions.

An intriguing factor appears relevant to the excellent performance ofSrI₂(Eu). That is, the lattice constants for SrI₂ and EuI₂ are nearlyidentical, thus permitting high doping of Eu in SrI₂. Other favorableaspects of SrI₂ include its tow melting point, 538° C., and itsorthorhombic crystal structure, which will likely be readily grown tolarge sizes.

The alpha particle-induced luminescence of BaI₂(Eu) is similar to thatof Lu₃Al₅O₁₂(Ce), but shifted to shorter wavelength.

A marked advantage of using SrI₂ or BaI₂ crystals doped with Eu is therelative ease in which the crystals can be grown in large sizes. Anotheradvantage is that SrI₂(Eu) and BaI₂(Eu) are less hygroscopic thanCaI₂(Eu) which is an important practical edge in using Sr or Ba insteadof Ca.

In a first general embodiment, a material comprises a crystal, which iscomprised of strontium iodide (doped or undoped) providing at least50,000 photons per MeV. In one particularly preferred embodiment, theenergy resolution of the crystal may be less than about 5.0% at 662 keV,as being enhanced by doping, e.g., with Ce or Eu.

In the first general embodiment, the crystal may be doped with europiumin different percentages, such as containing more than 1.6% europium,containing between about 0.5% and about 8.0% europium, and containingmore than 2.0% europium.

In addition, the europium in the crystal may be primarily Eu²⁺. The useof Eu²⁺ surprisingly provides excellent energy resolution, e.g., lessthan about 2.7% at 662 keV, As noted above, conventional wisdom and aprevious report indicated that such energy resolution was impossible forsuch a material. To exemplify, FIG. 8 is a chart 800 depicting energyresolution as a function of gamma ray energy acquired for SrI₂(5% Eu)and LaBr₃(Ce) crystals. These crystals are the same as used below inExample 4. As shown, the energy resolution of SrI₂(5% Eu) is comparableor slightly better than that of LaBr₃(Ce).

Another variation of the first general embodiment is where the crystalhas at least one dopant, selected from: cerium, praeseodymium, thallium,or lead,

The first general embodiment may further include barium in the crystal,or the crystal may provide at least 60,000 photons per MeV. Further, theresolution of the crystal may be less than about 5%, less than about4.0%, etc, at 662 keV.

In a second general embodiment, a scintillator radiation detectorcomprises a scintillator optic comprised of strontium iodide (doped orundoped) providing at least 50,000 photons per MeV. In one particularlypreferred embodiment, the energy resolution of the crystal may be lessthan about 5.0% at 662 keV.

Also, in the second general embodiment, the scintillator optic maycontain more than 1.6% europium, may contain between about 0.5% andabout 8.0% europium, or may contain more than 2.0% europium. Inaddition, the europium may be primarily Eu²⁺. Further, the scintiliatoroptic may include barium and/or calcium.

With continued reference to the second general embodiment, thescintillator optic may provide at least 60,000 photons per MeV, and mayhave a resolution of less than or about 4.0% at 662 keV.

In a third general embodiment, a scintillator radiation detectorcomprises a scintillator optic comprised of SrI₂ and BaI₂, wherein aratio of SrI₂ to BaI₂ is in a range of between 0:1 and 1:0.

In the third general embodiment, the scintillator optic may provide atleast 50,000 photons per MeV and energy resolution of less than about5.0% at 662 keV,

Further, the scintillator optic may contain europium, and the europiummay be primarily Eu²⁺. In addition, the scintillator optic may provideat least 80,000 photons per MeV, and may contain at least one dopant,selected from: cerium, praeseodymium, thallium, or lead.

In a fourth general embodiment, a scintillator radiation detectorcomprises a scintillator optic comprising barium iodide.

In the fourth general embodiment, the scintillator optic may be dopedwith at least one of cerium, praeseodymium, thallium, lead, indium, or atransition metal ion. Also, the scintillator optic may be doped with anactivator that luminesces in response to gamma radiation.

In the fourth general embodiment, the activator may include an ion whichluminesces via a 5d→4f transition or the activator may include an s² ionor a closed shell ion. Further, the activator may be a transition metalion.

In a fifth general embodiment, an iodide crystal comprises a singlemetal ion (M, M′ or M″) with the formula MI₂, M′I₃, or M″I₄, where M orM′ has an atomic number >40, but is not Y, Sc, La, Lu, Gd, Ca, Sr or Ba.M″ may or may not have an atomic number greater than 40.

Any of the general embodiments may include further limitations asdirected below. In addition, combinations of the additional limitationsdirected below may be combined to create even more permutations andcombinations of features.

EXAMPLES

To demonstrate various embodiments of the present invention, severalexamples are provided bellow, It should be appreciated that these arepresented by way of nonlimiting example only, and should not beconstrued as limiting.

Example 1

Strontium iodide and barium iodide crystals were grown in quartzcrucibles using the Bridgman method. The melting points of SrI₂ and BaI₂are 515 and 711° C., respectively; both possess orthorhombic symmetrywhile calcium iodide is hexagonal. All crystals described in thissection were doped with 0.5 mole % europium and were several cubiccentimeters per boule, then cut into ˜1 cm³ pieces for evaluation.Barium iodide as-supplied powder, 99.995% pure ultradry (Alfa Aesar) wasyellowish in color (thought to be due to oxide or oxyiodidecontamination). Crystals grown directly from as-supplied powdersretained a dark coloration (referred to henceforth as “first crystal”),Zone refining rendered the starting powders colorless, and the resultingpure powders were used to grow several crystals (referred to as “secondcrystal,” although several were grown following this procedure).Finally, an ultrapurificafion method was used to grow a BaI₂(Eu)crystal, referred to as “third crystal.”

Radioluminescence spectra were acquired using a ⁹⁰Sr/⁹⁰Y source (averagebeta energy ˜1 MeV) to provide a spectrum expected to be essentiallyequivalent to that produced by gamma excitation, Radioluminescencespectra were collected with a spectrograph coupled to athermoelectrically cooled camera and corrected for spectral sensitivity.The beta-excited luminescence of SrI₂(0.5% Eu) compared to that of astandard scintillator crystal, CsI(Tl), is shown in FIG. 1 in the upperplot 102, along with a SrI₂(Eu) crystal grown at RMD. It possesses asingle band centered at 435 nm, assigned to the Eu²⁺ d→f transition, andan integrated light yield of 93,000 photons/MeV. FIG. 1 in the lowerplot 104 shows beta-excited luminescence spectra of three BaI₂(Eu)crystals compared to a CsI(Tl) standard crystal. The Eu²⁺ luminescenceat 420 nm is enhanced in the second BaI₂(Eu) crystal, while the ˜550 nmband is reduced, and for the third crystal, the ˜550 nm band is entirelyabsent. It is notable that the overall light yield is highest for thefirst crystal; its integral tight yield (including both the 420 nm andthe 550 nm bands) is 60,000 photons/MeV. The weak band at 550 nm may beassigned to an impurity-mediated recombination transition.

Example 2

Calcium iodide and strontium bromide crystals were grown via theBridgman method, with 0.5% Europium doping. The CaI₂(Eu) crystal issubstantially opaque due to optical scatter, considered unavoidable dueto its platelet crystal structure. Its radioluminescence spectrum wasmeasured at 110,000 Ph/MeV, and is shown in the chart 200 of FIG. 2.SrBr₂(Eu) is an orthorhombic crystal with good optical properties,however, its light yield so far is low (˜25,000 Ph/MeV). AUradioluminescence spectra reported herein were acquired with a ⁹⁰Sr/⁹⁰Ysource (˜1 MeV average beta energy) and emission spectra were collectedusing a Princeton Instruments/Acton Spec 10 spectrograph coupled to athermoelectrically cooled CCD camera.

Example 3

Undoped strontium iodide was grown and zone-refined. The luminescencespectrum, shown in FIG. 2, is unchanged between pure and impure segmentsof the bottle, however, the pulse height spectrum of the purer sectionis slightly higher, Pulse height measurements, shown in the chart 300 ofFIG. 3, were acquired using a Hamamatsu R329EGP PMT (QE at 550 nm of15%). The signals from the PMT anode were collected on a 500 Ω resistor,shaped with a Tennelec TC 244 spectroscopy amplifier (shaping time of 8μs) and then recorded with the Amptek MCA8000-A multi-channel analyzer,The emission is likely due to self-trapped excitons, as it is presentfor all un-doped samples.

Example 4

A scintillator crystal of strontium iodide doped with 5% europium, ascintillator crystal of LaBr₃(Ce), and a scintillator crystal ofNiaI(Tl) were acquired and exposed to a ¹³³Ba source. Acquisitionparameters (e.g., shaping time, gain) were optimized for each crystal togive the best results for the particular crystal. The resulting gammaray spectra are shown in the chart 400 of FIG. 4A. As shown, the energyresolution of the SrI₂(Eu) is better than the energy resolution of theLaBr₃(Ce) in the low energy region.

Example 5

The same crystals used in Example 4 were exposed to a ¹³⁷Cs source,which is primarily monoenergetic. Again, acquisition parameters (e.g.,shaping time, gain) were optimized for each crystal to give the bestresults for the particular crystal. The resulting gamma ray spectra areshown in the chart 402 of FIG. 4B. As shown, the energy resolution ofthe SrI₂(Eu) is comparable to the energy resolution of the LaBr₃(Ce).

Example 6

The same LaBr₃(Ce) and SrI₂(Eu) crystals used in Example 4 were exposedto a ¹³⁷Cs source, which is primarily monoenergetic. Again, acquisitionparameters (e.g., shaping time, gain) were optimized for each crystal togive the best results for the particular crystal. The resulting gammaray spectra are shown in FIG. 4B. As shown, the energy resolution of theSrI₂(Eu) is comparable to the energy resolution of the LaBr₃(Ce).

Example 7

Several crystals of barium iodide were grown and characterized. Theradioluminescence of BaI₂(Eu) typically shows both a long-wave band,similar to that seen in undoped SrI₂, as well as the BaI₂(Eu) band shownin FIG. 2. The long-wave band, thought to be related to self-trappedexciton luminescence, is reduced as the Eu doping level is increased.However, even for crystals exhibiting only Eu luminescence, gamma lightyields and energy resolution so far are modest (see the chart 500 ofFIG. 5).

Example 8

Barium Bromide crystals were grown doped with Eu, but the light yieldsare <30,000 Ph/MeV. While it may be possible for the performance of BaI₂and BaBr₂ to be improved, but energetic considerations, such as relativepositions of the Eu²⁺ states within the bandgap, may limit light yields.For example, the Eu²⁺ excited state in BaI₂ may be too close to theconduction band to compete effectively with residual shallow traps,while this matter is resolved in SrI₂ since the Eu²⁺ excited state isslightly lower with respect to the conduction band.

Therefore, of the alkaline earth halides. SrI₂(Eu) appears mostpromising due to its very high light yield, good optical properties,ease of growth, high achievable doping with Eu²⁺, Z_(eff) higher thanLaBr3(Ce), excellent light yield proportionality and demonstrated energyresolution of <2.7% at 662 keV. CaI₂ has not been effectively grown inlarge sizes and SrBr₂ has a low Z_(eff), while BaI₂ and BaBr₂ have notdemonstrated adequate light yields for high energy resolution.

Example 9

Decay times were acquired using a flashlamp-pumped Nd:YAG laser usingthe 4^(th) harmonic at 266 nm, and 20 ns FWHM pulses. Luminescence wascollected with a monochromator coupled to an R928 Hamamatsu PMT and readout by an oscilloscope. in SrI₂(Eu), the Eu²⁺ band decays with a 1.2microsecond time constant as shown in FIG. 7, top plot 702. FIG. 7,middle plot 704, shows that for the BaI₂(Eu) crystal grown withas-received powder, the Eu²⁺ decay is about 450 ns while theimpurity-mediated luminescence is slower, and cannot be fully integratedwithin an 8 μs shaping time. It is interesting that a component of theimpurity-mediated recombination pathway is prompt (pulsewidth-limited)proceeding, directly by trapping carriers from the conduction andvalence band, but there is also a component that forms by depopulatingthe Eu²⁺ excited state (possibly electrons trapped initially at Eu²⁺ areable to thermally de-trap to the conduction band), as revealed by arise-time component observed for 600 nm detection. Also, theimpurity-mediated decay is very slow, on the tens of microsecondstimescale (perhaps due to an exciton experiencing a triplet to singletspin-forbidden transition). For ZR BaI₂(Eu), the Eu²⁺ decay is about 770ns, as shown in FIG. 7, bottom plot 706, effectively lengthened due tothe reduction of de-trapping and excitation transfer to theimpurity-mediated recombination pathway.

ILLUSTRATIVE METHOD

Now referring to FIG. 6, a method 600 according to one embodiment isshown. As an option, the present method 600 may be implemented in thecontext and functionality architecture of the preceding descriptions. Ofcourse, the method 600 may be carried out in any desired environment. Itshould also be noted that the aforementioned definitions may applyduring the present description.

With continued reference to FIG. 6, in operation 602, strontiumiodide-containing crystals are mixed with a source of Eu²⁺. Any type ofstrontium iodide-containing crystals may be used, and the source of Eu²⁺may be of any type.

In operation 604, the mixture is heated above a melting point of thestrontium iodide-containing crystals. The melting point may be differentthan that of Eu²⁺ alone or It may be different than a melting point ofstrontium iodide-containing crystals alone.

In operation 606, the heated mixture is cooled near the seed crystal forgrowing a crystal. The grown crystal may contain more than 1.6%europium, more than 2.0% europium, or between about 0.5% and about 8.0%europium. Further, the europium in the grown crystal may be primarilyEu²⁺.

IN USE

Embodiments of the present invention may be used in a wide variety ofapplications, and potentially any application in which high light yieldor high resolution is useful.

Illustrative uses of various embodiments of the present inventioninclude, but are not limited to, applications requiring radiationdetection. Search, surveillance and monitoring of radioactive materialsare a few such examples. Various embodiments can also be used in thenuclear fuel cycle, homeland security applications, nuclearnon-proliferation, medical imaging, etc.

Yet other uses include detectors for use in treaty inspections that canmonitor the location of nuclear missile warheads in a nonintrusivemanner. Further uses include implementation in detectors on buoys forcustoms agents at U.S. maritime ports, cargo interrogation systems, andinstruments that emergency response personnel can use to detect orsearch for a clandestine nuclear device.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

1. (canceled)
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. (canceled) 6.(canceled)
 7. (canceled)
 8. (canceled)
 9. (canceled)
 10. (canceled) 11.(canceled)
 12. (canceled)
 13. (canceled)
 14. (canceled)
 15. Ascintillator radiation detector, comprising: a scintillator opticcomprising SrI₂ and BaI₂, wherein a ratio of SrI₂ to BaI₂ is in a rangeof between 0:1 and 1:0, wherein the scintillator optic is a crystal thatprovides at least 50,000 scintillation photons per MeV and energyresolution of less than about 5% at 662 keV, wherein the scintillatoroptic contains more than 2% europium; and wherein the scintillator opticcontains at least one co-dopant, selected from cerium, praseodymium,thallium, or lead.
 16. The scintillator radiation detector of claim 15,wherein the crystal provides an energy resolution of less than about 4%at 662 keV.
 17. The scintillator radiation detector of claim 15, whereinthe scintillator optic contains more than 2% europium and less than 8%europium.
 18. The scintillator radiation detector of claim 17, whereinthe europium is primarily Eu²⁺.
 19. The scintillator radiation detectorof claim 15, wherein the scintillator optic provides at least 80,000photons per MeV.
 20. (canceled)